The present invention relates to a method and means for equalizing the measuring sensitivity of a plurality of strain gages connected in transducer bridge circuits for example for achieving uniform weight readings regardless of the position of a load on the platform of a scale. Specifically, the invention provides a new method and apparatus for equalizing the sensitivity of a plurality of beam type force transducers associated with platform scale transducers. The invention is particularly applicable to scales and devices for measuring mass or force in which the signal outputs are derived from strain gages forming part of force sensitive transducers located, for example, in a scale at the corners of the scale structure. The present method is advantageously applied to the gaged plate transducers and platform type scales described in the cross-referenced patent and patent applications.
U.S. Pat. No. 4,261,429 discloses a platform scale in which strain gage transducer beam elements are formed as integral components of the platform. The transducer beams have a flexural free end and a fixed end merging into the platform structure. For this purpose, slots are milled, machined, or cut into the platform so that two parallel slots define the beam structure. A cross slot may define the free end. The sensitivity of the beam is initially established by drilling holes through the platform at the fixed end of each slot and intermediate the ends of each slot. A plurality of transducer beams, typically four, are situated in the corners of the platform and the force associated with an applied mass or load may be applied to the transducer beams in various ways, for example by ball and socket loading means.
Strain gages are secured, for example, by adhesive bonding to the transducer beams. All the strain gages are electrically interconnected to form bridge circuits which in turn provide a combined signal output from the platform scale directly related to the sum or average of the forces measured by the respective strain gages and transducer beams. Structures employing beam type transducers constructed as an integral part of a platform are referred to herein as "gaged plate transducers" or simply as "gaged plates".
Such gaged plate transducers are incorporated in platform scales of U.S. Pat. No. 4,411,327 referred to above. Two parallel horizontal platforms are coupled together in the weighing apparatus. A load is placed on the upper horizontal platform. The strain gage transducer beams are integrally formed in the lower horizontal platform which constitutes a gaged plate transducer. Force transmitting bars or brackets in each corner transfer a vertical load from the upper platform to the respective flexural strain gage transducer beam structures in the lower platform through a variety of couplings. These couplings transfer vertical load components to the strain gage transducer beams while substantially reducing or preventing the introduction of transverse or lateral load components. According to one embodiment, a "ball and socket" type loading is used for substantially preventing the transmission of horizontal force components while at the same time transmitting vertical force components from the upper platform into the lower gaged plate transducer platform.
In the example of the gaged plate transducers it is practical to provide each of the four transducer beams with two strain gages secured as mentioned above. The eight strain gages are interconnected in two parallel bridge circuits providing a sum or average of the loads or strains measured by all the strain gages. It is desired that the combined signal output remains constant or uniform for any particular load regardless of the position of the load on the platform. To achieve such uniform output signal independently of any load position the sensitivities of the respective transducer beams must be equalized. In U.S. Pat. No. 4,261,429 the sensitivities of the respective strain gage transducer beams are made equal by mechanically adjusting or reducing the cross-sectional dimensions of a portion or portions of the transducer beam where the strain gages are secured. The thickness or width of the beams where the strain gages are located is reduced by filing or grinding until the sensitivities at all corners or rather of all transducer beams are equal within an acceptable tolerance range.
The procedure for achieving such uniform sensitivity by mechanical adjustments is as follows. The load is applied successively to each corner or other load sensitive transducer location of the gaged plate transducer. The scale output is measured at each successive position to determine the relative sensitivity of each transducer beam to the applied load. The outputs at the least sensitive locations are increased by filing or grinding thereby reducing the width or thickness of the transducer beam in the area where the strain gages are located. These steps may be repeated until the sensitivities of the respective transducer beams are equalized. Highly accurate gaged plate transducers and platform scales yielding load measurements which are quite independent of the position of the load may be provided by this method. However, the method has been found to be time consuming for a number of reasons.
Substantial grinding or filing causes heating of the transducer beams and therefore the temperature of the strain gages located near the area where mechanical material removal occurs is also raised. This is not desirable because the individual strain gages are temperature sensitive and conventional temperature compensation methods do not operate effectively because these localized elevated temperatures are not sensed by the remotely located temperature compensation components and errors may occur when attempting adjustment of output or sensitivity in each corner. It is therefore necessary for the operator carrying out the mechanical sensitivity equalization to allow the temperature of each beam to return to its ambient level before further load measurements and subsequent adjustments may be made. A considerable number of repeated and gradual adjustments are normally required for equalization of sensitivity in all scale corners. The total time required for the mechanical method of sensitivity adjustment may therefore be quite long because of the required temperature stabilization periods.
Adjustment of gaged plate sensitivity in each corner by filing or grinding of the transducer beams is a laborious and time consuming task, for other reasons as well. To perform the filing the gaged plate is normally turned upside down and relatively large amounts of material may have to be removed from the flexural sensing portion of the transducer beam. The platform scale is then returned to its normal position and reassembled to the loading fixtures for further measurements with a load applied to different locations on the platform. The process may have to be repeated several times to equalize the sensitivity in each corner of a scale to within a small specified tolerance limit of, for example, 0.02%. The procedure may require up to two or three hours for a satisfactory equalization of sensitivities.
A further complication may attend the mechanical method of equalization of sensitivities when applied to the gaged plate transducers of the cross-references. Generally two strain gages are secured to each transducer beam at two sensing positions. The location of the strain gages and the manner in which they are electrically interconnected renders the beams and associated strain gages relatively insensitive to position of the load introduction to the beam. If equal amounts of material are not removed from the two sensing positions of the beam, the beam may become more sensitive to the point of load application on the transducer beam. Under normal conditions of equal cross-sectional width and thickness of the beam at the two strain gage locations, the sensitivity of the transducer beam is substantially the same regardless of the point of load application. But during the material removal at each strain gage location, it is difficult to maintain the two cross-sections the same. As a result, the sensitivity to load introducing position increases. Even though the ball and socket load introduction technique minimizes changes in load introduction positions there is unavoidably some change in load introduction positions under repeated loadings. As the beams become more sensitive to load introduction positions resulting from differing beam cross-sections, repeatability errors develop because of the small changes in load introduction positions encountered even with the ball and socket load introduction means.